"If that's the case, we should just be able to demonstrate it in a controlled experiment, like we can with gravity."
Correct, and scientists have demonstrated genetic change within a population (evolution) millions of times.
Very sloppily said.
Genetic change does not equal evolution.
My children, while genetically quite similar to me are still genetically different from me - my bloodline has changed in one generation.
Yet I would hesitate to call my daughter a different species from my wife.
In fact, the extreme conservatism of genetic change from generation to generation militates against the plausibility of macroevolution - hence the stopgap arguments like "punctuated equilibrium" etc. to temper the extremism of evolutionary claims.
And yet, after thousands of generations of changing their diets, bombarding them with radiation, altering their atmospheres, and so on, fruitflies remain fruitflies.
As always, the evolutionist must confuse the distinction between micro-evolution (variations within a species, such as breeding dogs) with macro-evolution (breeding dogs until they become cats). The former is well-established--the latter is sheer speculation that is so unsupported by the fossil record that Stephen Jay Gould had to invent the theory of punctuated equilibrium to explain away the lack of transitional fossils.
When you have bred a dog into a cat, or a fruitfly into a hornet, let me know. Until then, claiming that (macro-)evolution is as established as gravity (which I just proved by dropping my pen and watching it fall to the floor--want to see me do it again?) is a statement of metaphysical faith, not one of science.